Synchronously Switched Active EMI Filter

A switch-mode active electromagnetic interference (EMI) filter (AEF) synchronized to a power converter is introduced in this article, with very low energy storage requirements compared to passive EMI filters and very high efficiency compared to typical AEF circuits with linear amplifiers. The proposed AEF does not use feedback like typical AEF circuits and thus avoids the bandwidth and attenuation limitations imposed by feedback stability. It can also benefit from simple circuitry by directly using the same gate signals as the main power converter. Moreover, the AEF does not interfere with the closed-loop controller of the main converter that is usually a challenge when designing a passive EMI filter or a feedback-based AEF. We demonstrate the proposed AEF with simulations and hardware prototypes to filter ripple current from a boost power factor correction (PFC) operating in both constant-frequency ACC and variable-frequency hysteretic control. The proposed AEF not only achieves high differential-mode (DM) current attenuation from
$20 \,\mathrm{dB}\,\mathrm{to}\, 40 \,\mathrm{dB}$
, but it also provides high common-mode (CM) current attenuation over
$29 \,\mathrm{dB}$
by injecting a CM current that largely cancels the CM current caused by the boost PFC. The efficiency penalty is low (
$< 0.3\%$
), even when filtering high ripple ratios up to
$\mathcal {R}=100\%$
. The proposed switch-mode AEF is a very promising approach to (mostly) replace the conventional
LC
filter and feedback-based AEF for smaller size and high efficiency. Last, we further extend the current cancellation technique for other converter topologies from buck, boost, buck–boost, SEPIC, Ćuk, etc., with an appropriate auxiliary active circuit to eliminate ac ripple current from input or output of the main converter.